Is the 64 Bit DAC truly 64 bits and why?

Yes the DAC is truly a 64 bit design featuring a 64 bit data path per channel to the input of the DAC. It features a 64 bit system to avoid truncation of bits, as all 16 bit inputs(a standard CD is 16 bits) create a 32 bit coefficient, and standard 24 bit DACs truncate this data path. (See the digital engine PDF for further technical information)